A. Field of the Invention
The present invention pertains generally to optics and more particularly to methods and apparatus for generating a uniform light source.
B. Description of the Background
Many different optical applications require the generation of light that has a uniform intensity over a predetermined spatial region. For example, a uniform source of light can be used to illuminate various objects for imaging or scanning applications. One specific example relates to the scanning of bar codes using an optical detector, usually a linear Charged Coupled Device (CCD) array.
A common way of illuminating targets such a bar codes for scanning purposes is to use a linear array of light emitting diodes (LED) or laser diodes, such as illustrated in FIG. 4. These sources of light, however, can vary greatly in intensity. For example, typical light emitting diodes can individually vary by as much as a factor of 4. To overcome this problem, selection processes are used to select LEDs that have substantially the same intensity output. However, these processes of selecting LEDs are expensive and substantially increase the cost of the resultant array. Though this method can be used, the forward current in each individual LED has to be accurate enough to obtain a uniform source.
Further, when mounting the LEDs in the array it is difficult to insure that each of the LEDs is aligned in a common direction. In the process of mounting these LEDs it is common for the LEDs to be slightly skewed or mounted off axis so that a uniform output is not provided. Techniques for insuring that each of these LEDs is properly aligned also substantially increases the cost of the LED array.
When illuminating a predetermined spatial region such as a bar code with a linear array, it is desirable to get a uniform signal at sensor output. Other problems exist in generating an intensity of light that gives an uniform signal at the output of the CCD such as lens vignetting, field attenuation that varies by the square of the distance along the length of the target, and even sensor sensitivity non-uniformity. These problems can be overcome, to some extent, by the use of compound lenses or other optics that are placed in front of the CCD sensor. However, compound lenses are expensive and normally cannot provide a complete correction of these problems unless they are custom designed for each individual application.
Further, light sources such as LEDs tend to age such that they produce a decreasing intensity over time. Initial calibration of an LED array will not necessarily result in a calibrated array after a period of time. Hence, the expensive calibration techniques that may be used to provide a uniform output intensity over a predetermined spatial region may be ineffective after a period of time especially since the individual sources of light may age differently. Other components in the system may also age which will also affect the ability to provide a source of light that is spatially uniform over a predetermined region.
It would therefore be desirable to have a system that is capable of calibrating the individual light source elements so that a uniform output can be achieved and therefore reduce the cost of the necessary optics and eliminate sorting procedures for selecting light elements having similar outputs. U.S. Pat. No. 5,382,782 issued Jan. 17, 1995 to Hasegawa et al. discloses a system for calibrating LEDs in an array. Hasegawa et al. discloses an iterative process of detecting and altering the output of an array in an attempt to provide a substantially uniform output. Adjustment of the elements is done by detecting where the luminance is higher and lower and then adjusting the LEDs in those particular areas. Some degree of uniformity can be achieved by repeating the process in an interative manner.
It would therefore be advantageous to provide a method of accurately adjusting the LEDs to provide a highly uniform output in a simple and easy fashion.
The present invention overcomes the disadvantages and limitations of the prior art by providing a method of calibrating an array of light sources to provide an accurate and uniform output light intensity over a predetermined spatial area.
The present invention may therefore comprise a method of calibrating an array of light sources to provide a substantially uniform output over a predetermined area comprising; sequentially activating each light source of the array of light sources; sampling light intensity over the predetermined area for each light source to generate a plurality of intensity signals; generating a normalized matrix of the plurality of intensity signals; inverting the matrix to produce an inverted matrix; calculating driver intensity data from the inverted matrix; using the driver intensity data to generate calibrated driver signals.
The advantages of the present invention are that a complete and accurate calibration of the light emitting diodes can be performed in a single step and in a very accurate manner that does not require multiple iterations. The response of the signal is substantially uniformly flat at the CCD output, so the control of the light source compensates any defects from the power source to the CCD output, including the optical path. This includes:
1. Variations in LED efficiency, including relative LED efficiency versus forward current, peak current and angular displacement,
2. Variations in LED forward voltage,
3. Variations in analog parts driving the LEDs,
4. LED off-axis tilt,
5. Relative LED off-axis positioning,
6. Optical focusing systems transmission defects,
7. CCD sensitivity, and
8. CCD sensitivity resulting from non -uniformity.
Since only electrical parameters can be controlled and analyzed within the system, the calibration procedures act on the electrical power control of each individual LED to provide an accurate analysis of the CCD output signal.
Also, light emitting diodes do not have to be sorted to match diodes having similar outputs. Standard diodes can be used that have not gone through a sorting process. No fine-tuning of the analog discrete parts associated with the LED forward current setting has to be done either. Further, special aligning techniques do not have to be used for mounting the LEDs in a linear array. The LEDs can be mounted in accordance with the standard mounting procedures that are accurate enough with respect to the calibration method used, and still be able to provide the desired uniform output. Further, expensive compound optics do not have to be used to generate a uniform output along the length of the target. By calibrating the system to adjust the intensities of the various light sources, expensive optical designs are not required. Additionally, even residual variations in the detected signal can be further adjusted by modulating the CCD array output signal with the inverse ripple signal that is used to adjust the sensitivity of the array. In this manner, a very accurate output can be produced, that gives the real reflectivity of the target which is analyzed, with a constant Signal to Noise Ratio across the field of view of the scanning device. This is required in most imaging applications.